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As we know that our body is made up mostly of water and the frequency of vibration of water molecules matches that of microwaves which is the working principle of microwave ovens.

When we come in contact of sunlight and feels its warmth we say that it's due to the heat waves coming as infrared. Atleast to explain the heat we feel shouldn't we give reasoning of microwaves rather than infrared ?

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It's a myth that microwave absorption by water is a resonant process. See Does a domestic microwave work by emitting an electromagnetic wave at the same frequency as a OH bond in water? for a discussion of this.

Light generally interacts with matter by interacting with the electrons in matter. Light has an associated oscillating electric field and this excites oscillations in the electrons in the matter. The electrons may in turn transfer their kinetic energy to lattice vibrations, with the end result of heating the object. Incidentally, black body emission is the reverse of this. Lattice oscillations scatter electrons causing transient dipoles, and these dipole oscillations generate the black body radiation.

How much heating occurs depends simply on how fast the radiation is absorbed, and this is highly dependant on the frequency of the light and the electronic properties of the material. For example silica glass absorbs very little in the visible region while graphite absorbs very strongly. You get similar variations in the microwave region. Water isn't actually a very strong absorber of the 2.45GHz microwaves used in domestic ovens, which is just as well otherwise the centre of the food wouldn't be heated.

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Perhaps you can shed a light on why IR over micro ? The involved principle may be different but micro still heats food more effeciently than other radiations ! – Rijul Gupta Jan 19 '14 at 9:59
@rijulgupta: microwave doesn't heat food more efficiently than other radiation. Well, it depends on what you mean by efficiently. A one kW visible light oven would transfer the same heat as a 1kW microwave oven, but because light is strongly absorbed it would cause fast heating of the surface of the food but leave the centre cold. However the average rate of temperature rise would be the same as for the microwave oven. – John Rennie Jan 19 '14 at 10:05
Still one doubt left, why IR over micro, for explanation of feeling heat in sunlight or other light sources ? – Rijul Gupta Jan 19 '14 at 10:08
@rijulgupta: if you put your hand in a working microwave oven it would feel hot! However this isn't the only factor. Visible light and IR are strongly absorbed in the skin, and this is where most of the nerve endings are concentrated. So a given intensity of visible/IR would feel hotter than the same intensity of microwave because more of it would be absorbed in the vicinity of your nerve endings. But both would end up depositing the same amount of heat in you. – John Rennie Jan 19 '14 at 10:17

"As we know that our body is made up mostly of water and the frequency of vibration of water molecules matches that of microwaves which is the working principle of microwave ovens".

Heat content of a body is the sum of the kinetic energy of all its molecules; average kinetic energy indicates the temperature. As you are saying frequency of vibration of water molecules in our body matches with that of microwaves, our body will be having at least the heat content produced by those of water molecules. So, our body will feel more hotness if it is incident by the waves which have greater frequency than that of microwaves. As we know, infra-Red rays have greater frequency than microwaves, so infrared ray's (or any other electromagnetic waves of higher frequency) gives us more heat than micro waves.

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